1. Field of the Invention
The present invention relates generally to a probe unit for contacting an electronic circuit such as an integrated circuit and, more particularly, to a probe unit having probes arranged in a pattern matching the contact pads of the integrated circuit.
The probe unit can be used for connecting the contacted integrated circuit that can be a memory device to an external device, e.g. a testing device with.
2. Description of the Related Art
In the semiconductor industry, the fabrication of monocrystalline monolithic integrated circuits involves their formation on a single microelectronic substrate formed on a silicon wafer. The circuits are applied to the wafer by lithography (e.g. photolithography, electron, ion or x-ray lithography), each wafer containing an array of identical integrated circuits. The wafer is then sliced into dies, so that each die or chip carries a single semiconductor device. To assure that the various circuits are functioning properly, several testing operations are performed after each stage of fabrication, usually at wafer stage using a wafer probe, and in some processes also using a bare die probe, and again after packaging using a packaged die probe.
Each semiconductor wafer includes many dies, typically several hundred. A typical wafer probe card is such as that described in U.S. Pat. No. 4,382,228. A conventional test probe unit includes an array of metal needles, or probes, in a fixed ring, which is mounted and electrically connected to a printed circuit board. Each needle has to engage a respective contact pad on the device being tested. However, when the density of contact pads of the device is high, a slight imprecision will cause the needles touching each other, thereby rendering the unit defective. Besides, manufacturing of probe unit of this type is very complex and requires alignment means.
Silicon chips are usually encapsulated in a plastic or ceramic package. However, the packaging of a chip is relatively costly and time consuming, the package greatly reduces the circuit density and degrades the electrical performance of the semiconductor device. At present, there is a strong drive towards the use of a bare silicon die without any package. As a significant number of chips may be defective, it is extremely important to test the bare die before installing it in a working printed circuit board.
A probe unit may require making contact with 1000 to 1600 contact pads (4 to 200 contact pads in each die, depending on die type, testing 16, 32, 64, 128, 256 die simultaneously). Since all the contact pads on the wafer lie in a common plane and must be contacted simultaneously, it is essential that all tips or points of the probes lie in a plane parallel to the common plane of the wafer and match die contact pads.
A test probe unit capable of testing an unpackaged die has been disclosed in U.S. Pat. No. 4,975,638. This patent describes a test assembly including a rectangular frame and a contactor fabricated from a dielectric flexible film with planar portion which sags below the frame. The probes are formed in the planar portion of the contactor. The test probe unit is adapted for contacting closely spaced contact pads arranged in a common plane, however only a limited number (typically, 4) of dies can be contacted for testing simultaneously. Moreover, displacement in Z axis causes corresponding displacement in X and Y axis, thus resulting in relatively low arrangement accuracy of the test assembly. The elasticity of flexible film is nonuniform along the film length and decreases with time. Manufacturing of the test assembly is complex and expensive.
A test probe unit disclosed in U.S. Pat. No. 5,103,557 comprises a baseplate made e.g. of a flexible silicon dioxide, a plurality of microscopic probe points arranged in the same pattern as contact pads of the tested device, connectors for connecting the probes to a testing device and conductors for providing electrical connection between the probes and the connectors. The contact points are the highest raised surface features having a hard metal core, a compressible portion, and a tip optionally plated with gold. The compressible probe points accommodate the height variances of the various dies, thus showing the independent height adjustment capability of each probe point which can be as much as 40% of its length and providing uniform pressure for all closely spaced probe points. However, the prior art unit has a relatively high cost due to the complexity of its design and multistage character of the manufacturing process. Moreover, the maintaining of the probes in their working condition when in use is complex and requires repeating the manufacturing steps.
Known are probe cards made on semiconductor substrates comprising a plurality of interconnection probes (EP 0755071 filed Jul. 18, 1996). A semiconductor device used as a probe card comprises a semiconductor substrate and a plurality of probe points formed as ball-bumps in recesses on the surface of the substrate Another wafer probe card is made of a silicone wafer having curved microtips formed on said silicone wafer is described in WO97/21105 of Jun. 12, 1997. An interconnection structure comprising a semiconductor substrate with a plurality of probes forming insertion structure has been described in WO 94/09513 of Apr. 28, 1994. The process of manufacturing these probe cards is complicated and includes multiple steps to form the projections, e.g. etching and lithography, involving the use of chemicals that makes the probe card expensive and process of manufacturing the probe card non-environment-friendly.
Therefore, it is an acute necessity to create a relatively cheap and easily maintained probe unit for an in-process testing a bare die using environment friendly technique that avoids chemical pollution of the atmosphere.
It is an object of the present invention to provide a simple and environment-friendly method of manufacturing a relatively cheap and easily-maintained probe unit having a plurality of probe points closely and accurately arranged in a common plane.
Another object of the present invention is to provide a probe unit wherein probe contacts lie in a common plane and correspond to contact pads of the semiconductor device with accuracy of up to xc2x11 micron.
According to the present invention, there is provided a method for producing a probe unit for contacting an electronic circuit having a predetermined pattern of contact pads deployed in a common plane, the method comprising the steps of making a test base plate of a material capable of surface uplift when irradiated, determining locations corresponding to said contact pads, irradiating the base plate at the determined locations until conical surface uplifts are formed; and providing means for electical connection between said conical surface uplifts and an external device.
Preferably, the surface irradiation is carried out by means of a laser.
The form of the microcontacts may be varied greatly by means of at least one of the parameters selected from the irradiation power density, irradiation impulse duration and/or dimensions of the irradiation zone. In one of the preferred embodiments the impulse energy source is a neodymium laser having a wavelength of 1054 nm, laser power density 7 kW/cm2.
According to another aspect of the present invention, there is provided a method for producing a probe unit for contacting an electronic circuit having a predetermined pattern of contact pads deployed in a common plane, the method comprising the steps of making a base plate of a dielectric material plated with electrically conductive material capable of surface uplift when irradiated, determining on the plated surface of the base plate locations corresponding to said contact pads, irradiating the plated surface of the base plate at the determined locations until conical surface uplifts are formed; and providing means for electical connection between said conical surface uplifts and an external device.
According to another aspect of the present invention, there is provided a probe unit produced by employing the above method.
According to still another aspect of the present invention, there is provided a probe unit for contacting an electronic circuit having connector pads deployed in a predetermined pattern in a common plane, the probe unit comprising a base plate made of a material capable of surface uplift when irradiated, a number of cone-shaped surface uplift microcontacts integrally projected from the base plate at irradiated locations corresponding to said connector pad pattern of the semiconductor device; and means for electical connection between said conical surface uplift microcontacts and an external device.
Typically, center-to-center spacing between probe points of the probe unit is as much as twice the spacing between contact pads of the die, the latter being usually the same as the contact pad width. The center-to-center spacing may be, for example, 120 microns for pad width of 60 microns. This spacing may be obtained by adjusting the center-to-center spacing of the irradiation zones and may be varied in the course of manufacturing process. Usually, this spacing may also be calculated as not less than 2 h, preferably not less than 4 h, more preferably not less than 6 h, where h is the desired contact height.
Preferably, the height h of the surface uplift microcontacts is in the range from 4 micron to 100 microns, preferably from 15 microns to 25 microns.
Preferably, the base plate is made of a dielectric material, and the surface uplift microcontacts are coated with conductive material.
According to still another aspect of the present invention, there is provided a probe unit for contacting an electronic circuit having connector pads deployed in a predetermined pattern in a common plane, the probe unit comprising a base plate made of dielectric material plated with electrically conductive material capable of surface uplift when irradiated, cone-shaped surface uplift microcontacts formed from said electrically conductive plating; and means for electical connection between said conical surface uplift microcontacts and an external device.
Preferably, the base plate is an integrated circuit chip, the irradiated locations are chip connector pads, and the means for electical connection are conductive traces on the surface of the integrated circuit.
Preferably, the plating comprises at least two layers, of which the inner layer is nickel, and the outer layer is gold.
According to still another aspect of the present invention, there is provided a probe unit for contacting an electronic circuit having connector pads deployed in a predetermined pattern in a common plane, the probe unit comprising a base plate made of a dielectric material, a number of surface uplift microcontacts formed on flexible elements mounted on the base plate; and means for electical connection between said conical surface uplift microcontacts and an external device, wherein the flexible elements comprise a material capable of surface uplift when irradiated, the surface uplift microcontacts are plated with a conductive material, and the means for electical connection are conductive traces on the dielectric base plate.
Preferably, the plating comprises at least two layers, the inner layer being nickel, and the outer layer being gold.
Preferably, the flexible elements are made of the material capable of surface uplift when irradiated.
Preferably, the flexible elements are plated with a material capable of surface uplift when irradiated.
The conductive material may be selected from metals and alloys providing the overall electrical resistance of less than 0.1 Ohm. Typically, the contact material may be selected from a group including copper, aluminium, gold or non-oxidising metals, such as non-oxidising precious metals. In some cases, to reduce the cost of a conductive layer and to enhance the adhesion to the surface of the dielectric material, precious metals may be used as the upper plating layer only, the inner layer being formed from metals having high adhesion to dielectric materials. The inner layer metals may be selected from the group including, but not limited to, nickel, tungsten, chromium, titanium, palladium, beryllium-copper alloy, or combinations thereof. The primer inner layer may be plated on the surface microcontacts formed on a dielectric base plate.
The dielectric material may be selected from the group including, but not limited to, silicon, silicon dioxide, silicon nitride, germanium, germanium dioxide, indium antimonide, nickel phosphide, gallium alloys, as well as some metal compounds and plastics capable of surface uplift being subjected to irradiation. Dielectric materials having maximum difference between liquid and solid state density are preferable.
Several probe units of the invention can be formed on a single base plate for simultaneously testing a plurality of dies. A significant advantage of the invention is that the probe contacts lie in a common plane and correspond to contact pads of the semiconductor device with accuracy of up to xc2x11 micron. The proposed invention avoids using complex means for maintaining the contacts in a common plane and reduces the time required for positioning the probe points in one plane in the course of testing procedure, thus increasing the test speed and total manufacturing yield.